The present invention relates generally to phased array antenna systems and more particularly to a multi-element adaptive antenna array designed to decrease mutual coupling between the antenna elements.
The Global Positioning System (GPS) includes a plurality of GPS satellites which transmit L band radio frequency (RF) signals that provide a geodetic reference to recipients. The GPS system requires full hemispherical coverage and receiving antennas should have elements designed to provide enhanced low angle coverage close to the horizon. When this type of element is used in an adaptive array of identical elements, this low angle coverage results in substantial coupling between the reference element and the adaptive elements. The adaptive elements are essentially resistively terminated with the resistive loads in the hybrid feed network and the low noise amplifier input impedance. The high mutual coupling between the reference element and terminated adaptive elements results in signal loss to the reference element.
This loss of signal could be rectified to some extent by moving the adaptive elements further from the reference element. However, this approach has two main disadvantages. The larger spacing could cause potential spurious nulls in the adaptive mode, and more important, it results in a larger, (and heavier) array that might not fit in the allocated space where space is limited.
The task of providing a multi-element adaptive antenna array with reduced mutual coupling is alleviated, to some degree, by the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 2,928,087 issued to Parker on 8 Mar. 1960; PA0 U.S. Pat. No. 3,967,276 issued to Goubau on 29 Jan. 1976; PA0 U.S. Pat. No. 4,246,585 issued to Mailloux on 20 Jan. 1981; PA0 U.S. Pat. No. 4,259,670 issued to Schiavone on 31 Mar. 1981; PA0 U.S. Pat. No. 4,260,994 issued to Parker on 7 April 1981; PA0 U.S. Pat. No. 4,466,003 issued to Royce on 14 Aug. 1984; and PA0 U.S. Pat. No. 4,431,998 issued to Finken on 14 Feb. 1984.
Parker U.S. Pat. No. 2,928,087 shows an omnidirectional beacon antenna having small height and diameter but with good gain in all directions. It includes a central radiating element and rotating elements, with coupling means including a transmission line to each rotating element. Parasitic modulating elements are spaced less than three half wavelengths from the center and the preferred number of them is nine. They may be quarter or half wave elements.
Goubau shows a four element antenna with top capacitor plates and printed circuit versions of having a central element and surrounding quarter wave conductors. In Parker U.S. Pat. No. 4,260,994 a plurality of arrays of parasitic antenna elements are used to synthesize and shape radiation patterns. Mailloux discloses adaptive nulling with an antenna array and Schiavone is directed to a broadband microstrip antenna.
The Finken patent discloses a GPS reference element composed of four sub-elements fed in phase rotation. A seven element adaptive array used seven of those reference type elements, each with four sub-elements. The center element in the seven element array was used as a reference element and the six identical elements surrounding it were used as adaptive elements. This configuration results in high mutual coupling between the reference element and the six adaptive elements with an associated loss in gain in the center reference element. This coupling reduces the efficiency of the center element when energy which should be received by it, is received by a neighboring element.
A possible solution to the mutual coupling phenomenon is to simply provide more spacing between the elements. However, in a variety of applications, this solution is not acceptable. For example, a limited amount of space is available for airborne conformal element arrays, and increased surface areas are not always available.
In view of the foregoing discussion it is apparent that there currently exists the need to reduce the mutual coupling experienced between antenna elements without significantly increasing the size of the antenna array. The present invention is intended to satisfy that need.